Summary of Work: We have cloned the large-conductance, calcium- and voltage-activated potassium channel (Bkca) that dominates membrane excitability in rat pituitary tumor cells (GH4C1) and studied its regulation in HEK cells, which were transfected transiently with the pore-forming alpha subunit. We have also studied the We have cloned the large-conductance, calcium- and voltage-activated potassium channel (Bkca) that dominates membrane excitability in rat pituitary tumor cells (GH4C1) and studied its regulation in HEK cells, which were transfected transiently with the pore-forming alpha subunit. We have also studied the inwardly-rectifying potassium channel (Kach) through which acetylcholine regulates excitability in acutely dissociated guinea pig atrial muscle cells because it is the system for which there is the most data supporting K channel regulation by direct G-protein binding. We have now demonstrated that ser/thr phosphatases, and not the betagamma subunits of G-proteins, are both necessary and sufficient for Kir regulation by acetylcholine in metabolically intact guinea pig atrial myocytes.